1. Field of the Invention
The present invention relates to a gear device and an electric power steering apparatus.
2. Description of Related Arts
A gear device for transmitting an output of a driving source such as an electric motor is known (for example, refer to the following document 1).
Document 1: Japanese Unexamined Utility Model Publication No. 05-45293
In this gear device, an output of an electric motor is transmitted to a load via a planet gear mechanism. To an output shaft of the electric motor, a sun gear is coupled. Between the sun gear and a ring gear, a planet gear is disposed. On the side surface of the planet gear, a concave groove for storing a lubricant is formed. Due to a centrifugal force associated with rotation of the planet gear (rotation and/or revolution), the lubricant in the concave groove is scattered. Thereby, the lubricant is spattered on the teeth of the gear.
However, to sufficiently spatter the lubricant on the surroundings by a centrifugal force, the rotational speed of the planet gear having the concave groove formed thereon must be high. Therefore, the driving conditions which enable sufficient lubrication are narrowed. For example, in a reduction gear mechanism which transmits an output of an electric motor to a steering mechanism, the rotational speed of a driven gear continued into the steering mechanism is comparatively slow (for example, several rpm). Therefore, it is difficult to sufficiently spatter the lubricant on the surroundings and sufficient lubrication is difficult. Particularly, when grease is used for lubrication, it is more difficult to scatter the grease by a centrifugal force.
The same problem occurs also in other general gear device as well as the gear device installed in an electric power steering apparatus.
The present invention is made against this background, and an object thereof is to provide a gear device and an electric power steering apparatus which realize sufficient lubrication under wider driving conditions.